Hair dyes can be classified by the dye to be used or by whether they have any bleaching action on melanin. Typical examples include a two-part permanent hair dye composed of a first part containing an alkali agent, an oxidation dye and optionally a direct dye such as a nitro dye and a second part containing an oxidizing agent; and a one-part semi-permanent hair dye containing an organic acid or an alkali agent, and at least one direct dye such as an acid dye, a basic dye or a nitro dye.
The above-described permanent hair dye is however accompanied with the drawbacks that the colour tone imparted by an oxidation dye is not so vivid and that the colour of the hair dyed with a vivid-colour producing nitro dye ordinarily employed as a direct dye markedly fades over time and becomes dull quickly even if the colour tone immediately after dyeing is very vivid (Japanese Patent Application Laid-Open (Kokai) No. Hei 6-271435).
It is common practice to combine direct dyes and oxidative dyes in permanent products to provide more vivid colour, however currently available direct dyes do not usually perform satisfactorily. The number of direct dyestuff that can be used in combination with oxidative dyes is limited by the necessity that they must be stable to alkaline peroxide during the dyeing process.
A variety of cationic direct dyes and nitro dyes have been used in permanent hair dye products to add brilliance and vividness to shades. However, in both cases the colour fades very quickly due to the loss of the direct dye from washing and light, especially on damaged or porous hair.